1. Rinse the rice under cold water until the water runs clear to remove excess starch and prevent a gluey texture.
2. In a medium pot, combine rinsed rice and 1 ½ cups cold water. Bring to a boil over medium-high heat, then reduce heat to low, cover with the lid slightly ajar, and simmer gently for 15 to 20 minutes or until rice is tender and water is absorbed. If rice is not fully cooked but water is gone, cover tightly and let it steam off heat for 5 to 10 minutes.
3. (If using leftover cooked rice, skip step 2.)
4. In a bowl, whisk together sugar and cornstarch to prevent lumps. Gradually add milk, whisking to combine. For added flavor, add cinnamon stick and vanilla bean pod during cooking or stir in vanilla extract after cooking.
5. Pour the milk mixture over the cooked rice in the pot. Place on medium heat and stir constantly until the mixture bubbles and thickens, boiling for 30 seconds to 1 minute or until the desired consistency is reached. Avoid instant rice as it behaves differently and does not yield the same creamy texture.
6. Remove from heat, discard cinnamon stick or vanilla pod if used. Stir in any optional mix-ins like raisins at this point.
7. Transfer to serving dishes. Serve warm or cover the surface directly with plastic wrap to prevent skin formation and refrigerate until cold, up to 1 day. For longer storage, the pudding can be frozen for up to 3 months though texture may slightly change upon thawing.